1. Field of the Invention
This invention relates to a process for manufacturing a part of a metal matrix composite material.
2. Description of the Related Art
A process for manufacturing a cylinder as disclosed in Japanese Patent Laid-Open Publication No. SHO-59-206154, for example, is known as a process for manufacturing a desired shape by plastic working from an aluminum-based composite material. It comprises:
(a) Dispersing SiC chips in a molten bath of aluminum under stirring and causing it to solidify;
(b) Heating a solidified product to a temperature of about 250 deg. C. and drawing it into a pipe; and
(c) Cutting a sleeve from the pipe, fitting it in a die casting mold and casting an aluminum alloy (JIS-ADC12) about it to make a cylinder.
The sleeve is a composite material obtained by putting SiC chips in a molten bath of aluminum and has a high resistance to plastic deformation, and its aluminum and SiC are mechanically joined to each other. Therefore, it is low in elongation and is as poor in workability as any other common composite material. As a consequence, it is difficult to employ for plastic working, such as drawing, to make a molded product of high quality at a reduced cost.
A part of a composite material can also be made by press forming, but a high resistance to plastic deformation brings about a high cost of production and makes it difficult to obtain a product of improved quality.
FIG. 17 hereof shows a disk brake device for an automobile. The disk brake device 202 has a brake disk 203 mounted by a hub 201 attached to the end of a drive shaft 200, and a caliper 206 in which an edge portion of the disk 203 is engaged. A hydraulic pressure is transmitted through a passage 208 to a cylinder not shown in the caliper 206 to press two brake pads 207 against the edge portion 205 of the brake disk 203 to thereby brake a wheel 209. Therefore, the brake disk 203 has to be formed from a material of high strength, while it is also desirable to use a light material to reduce the weight of the automobile.
A metal matrix composite material is known as a material of high strength and light weight. For example, a composite material containing an aluminum alloy can be used to achieve a weight reduction and a material containing SiC (silicon carbide) particles in an aluminum alloy makes it possible to achieve a high strength.
A brake disk 203 can be made by casting from such an aluminum-based composite material. A large amount of heat energy is, however, required for melting such a material and brings about an increase of production cost. Study has, therefore, been made of the possibility of relying upon press forming for manufacturing brake disks 203 in a large quantity without having to melt the material.
FIGS. 18A and 18B show a known method of manufacturing a brake disk from a composite material containing an aluminum alloy as a metal matrix. An aluminum-based composite material 210 is prepared in the form of a flat sheet, as shown in FIG. 18A, and is press formed into a brake disk 211, as shown in FIG. 18B. The brake disk 211 has a hub 213 having a recessed central portion and a flat disk portion 212 extending from the edge of the hub 213. The material 210, however, contains SiC particles which produce a relatively high frictional resistance in that portion of the material 210 which is contacted by a press whereby the disk 211 is formed, and the disk 211 is very likely to crack in its bent portions 214 and 215. Under these circumstances, it is difficult to manufacture a brake disk 211 by press forming from such a composite material.
It is an object of this invention to provide a process for manufacturing a part of high quality from a metal matrix composite material at a low cost.
According to a first aspect of this invention, there is provided a process comprising the steps of preparing an aluminum-based composite material containing an aluminum alloy and having an appropriate diameter, cutting the material into a plurality of blanks each having an appropriate thickness, heating the blanks to an appropriate temperature ranging from the solidus temperature, Ta, of the aluminum alloy minus 50 (Taxe2x88x9250) deg. C. to Ta deg. C., and press forming each blank, while holding it at the appropriate temperature.
When the temperature of the material is lower than (Taxe2x88x9250) deg. C., it has a high resistance to plastic deformation, is difficult to work on and requires a high working load. When its temperature exceeds Ta, a liquid phase may be formed and cause the material to crack during its plastic deformation because of its low compressibility. According to this invention, therefore, the temperature to which the blank is heated is at least equal to (Taxe2x88x9250) deg. C. to ensure its good workability, and does not exceed Ta deg. C. to ensure its good compressibility.
The aluminum-based composite material is prepared by reducing a porous reinforcing material composed of a metal oxide in a furnace containing a magnesium nitride atmosphere to expose a metal on at least a part of the reinforcing material, and impregnating the porous material with a molten aluminum alloy. The reduction of the metal oxide forms a metallized surface on the porous material and thereby produces an improved wetting property between the metal oxide and the molten aluminum alloy. The composite material is of high workability, since the aluminum and reinforcing material are strongly joined to each other by chemical contact. It is easy of plastic working and enables a reduction of production cost.
The die may have a heater for holding the blank at the temperature between (Taxe2x88x9250) deg. C. and Ta deg. C., so that the blank may be high in workability and easy to press form into a desired shape. The blank temperature is preferably in the range of (Taxe2x88x9220) to Ta, or for example, from 563 to 583 deg. C. to ensure that it be easy to work on to thereby enable a reduction of production cost. Moreover, it is preferably in the range of (Taxe2x88x9240) to (Taxe2x88x9233), or for example, from 543 to 550 deg. C. to ensure that the blank be of high compressibility to thereby attain a high working accuracy.
According to a second aspect of this invention, there is provided a process comprising the steps of preparing a die and a dual punch having a solid cylindrical inner punch portion and a hollow cylindrical outer punch portion surrounding the inner punch portion, setting a blank of an aluminum-based composite material on the die, lowering the inner punch portion to press against the central portion of the blank and holding it thereagainst to give a nearly final shape to the central portion of the blank and lowering the outer punch portion to press against the remaining portion of the blank to give a nearly final shape thereto.
The process includes two press forming steps in which a dual punch is used to form a blank of an aluminum-based composite material into a disk-shaped part. Firstly, the inner punch portion is pressed down against the central portion of the blank to form it into a desired shape, while drawing out the composite material uniformly from the center of the blank to give a high working accuracy to its central portion. Then, the inner punch portion is held against the central portion of the blank and the die is closed. Secondly, the outer punch portion is pressed down against the remaining portion of the blank to draw it out or cause it to flow in the die to form the composite material into a nearly final shape. As a result, there is obtained a product which requires only a small amount of machining work thereafter and thereby contributes to a reduction of production cost.
The outer punch portion forced into the closed die compresses the composite material therein by applying a uniform compressive force to the outer surface of the material at right angles thereto, so that it may be possible to reduce any tensile stress on the surface of the blank, prevent its surface from cracking, remove any internal defects from it and give it a tight structure to thereby make a part of improved quality.
The first press-forming step employing the inner punch portion may be used to form, for example, the boss portion of a crank damper pulley. The boss portion is easy to form by press forming from an aluminum-based composite material if the inner punch portion has an appropriately shaped surface.
According to a third aspect of this invention, there is provided a process comprising the steps of forming a sheet of an aluminum alloy covering the whole surface of a sheet of a metal matrix composite material to prepare a sandwiched structure having an aluminum alloy layer on both sides of the composite material, pressing the central portion of the sandwiched structure to form a recess therein, and removing the aluminum alloy layer from the remaining portion of the structure surrounding the recess.
A sheet of an aluminum alloy is formed to cover both sides of a sheet of a metal matrix composite material to prepare a sandwiched structure and the sandwiched structure is pressed in its central portion to have a recess formed therein. The aluminum alloy is high in workability and can, therefore, be shaped to cover both sides of the composite material to reduce any frictional resistance shown by the composite material during press forming, so that it may be possible to reduce any stress produced in the material and prevent it from cracking. The aluminum alloy layers are removed from the remaining portion of the material surrounding the recess to expose the metal matrix composite material to thereby enable the manufacture of a part of high strength. Thus, the process may most advantageously be employed for making a disk of a metal matrix composite material for a disk brake for an automobile.
The metal matrix composite material may be prepared by incorporating ceramic particles into an aluminum alloy. The aluminum alloy used as the matrix contributes to a reduction in weight of the composite material and the ceramic particles contribute to improving its strength. Thus, it is the most suitable material for brake disks.